The butterfly valves in existence today are usually built on the principle of soft gaskets. A very frequent valve type is illustrated in e.g. the Swedish patent specification No. 199 078. With this valve type the whole of the valve housing is lined with a soft material, e.g. rubber or other polymer. A frequent thing is also to manufacture just the valve seat itself from a soft material. Examples of this valve type are illustrated in the Swedish patent specifications Nos. 175 149 and 178 131. There are also cases of the soft sealing element being instead placed on the sealing face of the throttle. Examples of this principle are shown in the Swedish patent specification No. 195 072 and the German patent specifications Nos. 1 011 683 and 1 232 422.
A disadvantage of soft sealing elements in butterfly valves is that their resistance to high-temperature media is often low. It is true that the insensitiveness of synthetic rubber and certain other polymers to high temperatures has gradually improved, but still these kinds of material cannot in any way compare favourably with the resistance to high temperatures of steels and other metals and alloys. The same thing also applies to the resistance to certain chemically aggressive media. In these cases, too, the properties of highalloyed stainless steels and other alloys are quite superior to those of soft materials of rubber type. These circumstances are of course well-known, and many a trial has been made with replacing the soft sealing elements by metallic ones. In these cases, however, the sealing has not been to satisfaction or has the sealing device and/or the equipment parts belonging to it become so complicated that this valve type has not got any practical importance so far. An example of a valve belonging to this category is shown in the Swedish patent specification No. 193 923.